The present invention relates to lasers, and more particularly to systems and methods for controlling laser output frequency.
Fiber optic technology is finding increasing application in carrying growing amounts of data communication traffic including traffic related to operation of the Internet. With the growing use of the Internet to carry voice and video communications, the need for greater bandwidth will only expand further. Fiber optic technology offers the potential for vast increases in available bandwidth. As the cost of optical components decreases, fiber optic technology is spreading from the backbone to metropolitan networks and even to residential and business access lines.
Typically, an optical signal to be carried across a fiber is generated by a coherent light source, i.e., a laser. Amplitude modulation of a laser output is used to convey payload data down the fiber. This modulation may be applied either by direct variation of an output amplitude control signal fed to the laser or by use of a modulation component that processes a laser output.
There are limits, however, to the data rate that can be comfortably accommodated by a single optical signal as would be generated by a laser. For example, very high data rates, e.g., on the order of 40 Gbps or higher may be difficult to process electronically prior to modulation of a light signal. A technique for increasing the data carrying capacity of a fiber without increasing the modulation rate on a single optical signal is wavelength division multiplexing (WDM). In WDM, multiple optical signals having different wavelengths and each generated by a different laser are transmitted concurrently over a common optical fiber thus expanding the available data carrying capacity and providing other advantages in implementation. One current trend in WDM technology is closer spacing of multiple optical signals in the frequency domain. This allows for a greater payload data rate to be accommodated within a segment of the optical spectrum sufficiently narrow to lie wholly within the optimal communication band of the fiber optic link.
However, as the data rate carried by each individual optical signal increases, the occupied bandwidth also increases. The sidebands of adjacent optical signals may be very close to one another when the lasers operate at their nominal frequencies. Due to these close spacings, laser frequency drift cannot be tolerated. Furthermore, receiver components such as demultiplexers will assume that the optical signals remain close to their nominal positions in the frequency domain. Accordingly, there is a need to measure and control the frequency of each laser to avoid the possibility of one wavelength component drifting to the point that it overlaps another and also to assure proper receiver operation.
In a typical laser frequency control scheme, a four-port coupler is arranged so at to receive optical energy from a laser output and tap off a small portion to be fed through a first frequency-selective filter to a first photodetector. The response of the first frequency-selective filter has a slope such that the photodetector output signal increases with increasing laser frequency.
Signal energy that does not pass through the first frequency-selective filter reflects back into the coupler where it is directed to a second frequency-selective filter to which a second photodetector is connected. The second frequency-selective filter has a frequency response with a slope reversed compared to that of the first frequency-selective filter. A ratio of the outputs of the two photodetectors indicates the current laser output frequency and from this an error signal may be formed for frequency control.
A drawback of this scheme is that two photodiodes and two filters are required for each laser. As the number of channels in WDM systems continues to increase, it is desirable to minimize the optical components required to generate the signal on each channel. This includes the components for laser frequency control.